Do not redefine _GNU_SOURCE if already set

Or else, compilation will fail.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

srcpos.c

@@ -3,7 +3,9 @@
  * Copyright 2007 Jon Loeliger, Freescale Semiconductor, Inc.
  */
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 
 #include <stdio.h>
 

tests/testutils.c

@@ -5,7 +5,9 @@
  * Copyright (C) 2006 David Gibson, IBM Corporation.
  */
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE /* for strsignal() in glibc.  FreeBSD has it either way */
+#endif
 
 #include <stdio.h>
 #include <stdlib.h>